1. Field of the Invention
The present invention relates to a process for producing 2,6-dimethylnaphthalene and a process for producing a 2,6-naphthalenedicarboxylic acid. More particularly, it pertains to a process for producing highly pure 2,6-dimethylnaphthalene which is useful as a starting raw material for a 2,6-naphthalenedicarboxylic acid and the like in a high recovery rate in an industrially advantageous manner by means of an crystallization treatment for the mixture of dimethylnaphthalene isomers, and a process for producing a 2,6-naphthalenedicarboxylic acid from the 2,6-dimethylnaphthalene obtained by the aforesaid process which acid is useful as a starting raw material for a high performance polyester and the like.
2. Description of the Related Art
It has heretofore been known that a 2,6-naphthalenedicarboxylic acid and an ester thereof are each a compound of industrial importance as a starting raw material for a high performance polyester which is employed for the production of polyethylene naphthalate in the form of fiber, film and the like that is excellent in tensile strength and heat resistance.
The 2,6-naphthalenedicarboxylic acid and an ester thereof that are used for such a purpose are required to be highly pure and besides 2,6-dimethylnaphthalene (hereinafter dimethylnaphthalene is sometimes abbreviated to "DMN") which is used as a starting raw material thereof is required to be also highly pure because of the reasons described hereunder.
Specifically, 2,6-DMN, when being low in purity, causes the impurities contained therein to be oxidized or esterified, and eventually lowers the purity of a 2,6-naphthalenedicarboxylic acid and an ester thereof when being produced therefrom. A part of the impurities that are formed during the steps of oxidation and esterification, originating from the impurities in 2,6-DMN is extremely difficult to remove, thereby making it also extremely difficult to obtain a 2,6-naphthalenedicarboxylic acid or a dimethyl 2,6-naphthalenedicarboxylate each having high purity. In addition, such impurities, when being present in 2,6-DMN, deteriorates not only the purity of the above-mentioned acid and ester, but also the yield thereof based on 2,6-DMN to a remarkable extent. It is therefore, indispensable that highly pure 2,6-DMN be obtained in order to produce a 2,6-naphthalenedicarboxylic acid and a dimethyl 2,6-naphthalenedicarboxylate under industrially advantageous conditions. DMN has 10 isomers according to the positions of two methyl groups. Accordingly, 2,6-DMN as a starting raw material for a 2,6-naphthalenedicarboxylic acid is required to be a highly pure product substantially free from any of the isomers other than 2,6-DMN.
It being so, 2,6-DMN as a starting raw material for oxidation is desired to be highly pure as much as possible. In order to enhance the purity of 2,6-DMN, however, the burden imposed on its purification is increased. Hence, the purity of 2,6-DMN as a starting raw material for oxidation should be determined taking into consideration such factors as the burden on its purification, relation between the DMN purity and the result of the oxidation reaction, influence of its purity on the purity of the oxidation product and, in the case of purifying as a dimethyl 2,6-naphthalenedicarboxylate, relation between the DMN purity and the result of the methyl-esterification reaction and the burden on purifying the methyl ester in addition thereto. It is usually necessary to set the purity of 2,6-DMN on a high level of at least 98.0%, comprehensively taking these factors into consideration.
As the process for producing 2,6-DMN, there are available for example, a process in which 2,6-DMN is isolated from a tar fraction or a petroleum fraction, a process in which naphthalene or methylnaphthalene is methylated, succeedingly isomerized and separated and the like processes. Since the fractions and isomerization reaction products contain almost all of the 10 kinds of isomers, 2,6-DMN needs to be isolated from the mixture of a lot of isomers.
On the other hand, Japanese Patent Application Laid-Open Nos. 134634/1974, 8935/1975, 76852/1973 and 129534/1975 disclose a process for producing o-tolylpentene-2 in high yield from o-xylene and butadiene; a process for producing 1,5-dimethyltetralin by cyclizing o-tolylpentene-2; a process for producing 1,5-DMN in high yield and in high selectivity by dehydrogenating 1,5-dimethyltetralin; and a process for producing a mixture of isomers consisting essentially of 1,5-1,6- and 2,6-DMNs by isomerizing 1,5-DMN. Accordingly, by combining the above-mentioned processes it is made possible to produce a mixture of isomers consisting essentially of 1,5-, 1,6- and 2,6-DMNs from o-xylene and butadiene. Thus, there is made avialable a process for producing 2,6-DMN by isolating 2,6-DMN from the aforesaid mixture.
As described hereinbefore, any of the processes for producing 2,6-DMN that have heretofore been avialable makes it necessary to isolate 2,6-DMN from the mixture of isomers to recover the same. However, it is extremely difficult to purify 2,6-DMN by means of distillation which is frequently applied to the separation and purification of ordinary organic compounds, since 10 kinds of isomers have each a boiling point very close to one another as shown hereunder together with the melting point.
______________________________________ Boiling point (.degree. C.) Melting point (.degree. C.) ______________________________________ 1,5-DMN 269 82 1,6-DMN 266 -16 2,6-DMN 262 112 1,7-DMN 263 -14 1,8-DMN 270 65 2,7-DMN 262 98 1,3-DMN 265 -4.2 1,4-DMN 265 6 2,3-DMN 269 104 1,2-DMN 271 -3.5 ______________________________________
As is clear from the table, 2,6-DMN has a highest melting point of all the DMN isomers. On the other hand, it is known that 2,6-DMN forms a eutectic together with at least one of 1,5-DMN, 2,7-DMN and 2,3-DMN. It is therefore, necessary that the ratio by amount of 2,6-DMN in the isomer mixture to the isomers be more than the compositional ratio of the eutectic in order to precipitate 2,6-DMN as a crystal by means of crystallization from the mixture of the isomers. That is to say, the condition under which 2,6-DMN is at first precipitated as a crystal by cooling is that the molar ratios of 1,5-DMN, 2,7-DMN and 2,3-DMN each in the mixture of the isomers to 2,6-DMN in the same are not more than 1.9, 1.4 and 1.1, respectively.
As a method for isolating 2,6-DMN from the mixture of the isomers, there are proposed a crystallization method, an adsorption method, a method in which 2,6-DMN is caused to form a complex by the use of a certain kind of an organic compound, the resultant complex is separated and then it is decomposed to recover 2,6-DMN, and the like methods. Of these methods, crystallization method is most simple, convenient and suitable as an industrial isolation method.
In particular, in the case of producing a mixture of isomers consisting essentially of 1,5-, 1,6- and 2,6-DMN from o-xylene and butadiene as starting materials and isolating 2,6-DMN therefrom, a crystallization method is effective because of the comparatively small number of isomers in the starting raw material to be purified. In the case of methylating naphthalenes, isomerizing the reaction product and isolating 2,6-DMN or in the case of isolating the same from a tar fraction or a petroleum fraction, the combination of an adsorption method and a crystallization method is usually employed, since 2,6-DMN needs to be isolated from the mixture of a large number of isomers.
It is well known that in the case of isomerizing DMN, isomerization between adjacent .beta.-positions and isomerization of methyl-migration from one ring to another are unlikely to take place as compared with that between .alpha.-position and .beta.-position. Specifically, the above-mentioned 10 DMN isomers are classified into four groups, namely A to D groups as undermentioned with regard to isomerization, and isomerization among different groups is unlikely to take place as compared with that in the same group.
Group A--1,5-DMN, 1,6-DMN, 2,6-DMN PA1 Group B--1,8-DMN, 1,7-DMN, 2,7-DMN PA1 Group C--1,4-DMN, 1,3-DMN, 2,3-DMN PA1 Group D--1,2-DMN PA1 1,5-DMN/2,6-DMN&lt;1.9 PA1 2,7-DMN/2,6-DMN&lt;1.4 PA1 2,3-DMN/2,6-DMN&lt;1.1
As a method for isolating 2,6-DMN from a mixture of isomers by means of crystallization, there is disclosed in Japanese Patent Publication No. 22553/1975, a method in which the mixture is cooled to -10 to 20.degree. C. and the precipitated crystal is treated with methanol. The treatment with methanol is supposedly aimed at the improvement on the filterability of the crystal, but necessitates two-stage procedures. Japanese Patent Application Laid-Open No. 5767/1973 discloses a method in which a mixture of DMN isomers is washed or recrystallized by the use of an aromatic hydrocarbon. In this case, the mother liquor, which is formed after the separation of the crystal and contains the aromatic hydrocarbon, is usually isomerized to enhance the 2,6-DMN concentration and thereafter returned to the crystallization step. The aromatic hydrocarbon, when being contained in the isomerization procedure, which is usually carried out under a heating condition by using an acidic catalyst such as silica-alumina, zeolite and HF, causes partial disproportionation reaction to take place between DMN and the hydrocarbon, thus lowering the yield of the objective 2,6-DMN.
There is disclosed in the aforesaid Japanese Patent Publication No. 22553/1975 and Japanese Patent Application Laid-Open No. 5767/1973, that there is produced the objective 2,6-DMN having purity of 97.5 to 99.3%. However, any of the methods in the disclosures necessitates troublesome procedures and equipment therefor and requires a solid-liquid separation for the slurry obtained by crystallization in order to recover highly pure 2,6-DMN. In the case of industrial practice, the crystallization and solid-liquid separation need to be steadily carried out for a long period of time. Nevertheless, no description is so far found with regard to the change in the properties or purity of 2,6-DMN which is obtained in the case of continuously carrying out the crystallization and solid-liquid separation.
In addition, no detailed description is given of the means for separating the precipitated crystal from the mother liquor in any of the above-mentioned disclosures. In the case of separating the crystal from the slurry formed by crystallization for the sake of industrial practice, there are required simplified convenient equipment and procedures, stabilized running and a high separation efficiency. However, any description made from such industrial point of view is so far not found as to the means for isolating 2,6-DMN from the 2,6-DMN crystal-containing slurry obtained by crystallizing a mixture of DMN isomers.